Centrohelids: creatures of the sun

Sun worship is not unique to human societies. Evolutionary processes too seem to enjoy encountering sun-like shapes, from sea urchins to porcupine spines to radiolarians. Maybe not 'enjoy', perhaps, but a few of the encounters appear to have stuck around. Centrohelids were a member of the now-defunct Heliozoa" or "Sun-animalcules". The radial arrangement of fine rods (axopods) sticking out of the cell should suggest why. Heliozoa once included everything that looked sun-like, but, as often happens in the world of taxonomy, molecular sequencing put an end to that assemblage. While its constituents fell in various remote ends of the eukaryotic tree, its [arguably] flagship group remained intact, as centrohelid heliozoans -- or, to be politically correct now, centrohelids.

The entire cell is arranged radially around a central organising unit, the centroplast, from which the cytoskeletal framework of the axopod emerges. The cytoplasm is organised more or less concentrically -- immediately around the centroplast is an "exclusion zone" devoid of organelles, followed by a sphere of Golgi bodies, curiously oriented with their maturing end (where stuff comes out of) inward rather than outward. Around the Golgi layer is the endoplasmic reticulum as well as the nucleus tucked in between the axopods. The remarkable thing here is that a 'typical' cell is oriented with the nucleus near the centre, followed by the endoplasmic reticulum and then golgi bodies, maturing side facing outwards -- since generally you're trying to get secreted products to or near the surface. While seemingly perfectly shaped for this exact arrangement, the centrohelid cell is essentially inside-out, with the finished products delivered to the centre of the cell. But it makes sense -- at the centre, the secretion products hitch on to the axopod microtubules, and ride them outwards, to the edge of the cell body proper and beyond.

For the morbidly curious, most of these structures can be seen in the electron micrograph below from Bardele 1975 Cell Tiss Res, sectioned through the centre of the cell:

All this is surrounded by the ectoplasm, in this case a bubbly vacuolated layer where prey gets digested and savoured. And the cell body proper itself is usually covered by a myriad of spicules and plates, which can be seen in pictures here. This layer of surface decoration serves as a valuable feature for identifying species, to a certain extent. Of course, it may well be that what appear to be 'species' to our bloated macroscopic eyes may be genetically equivalent to entire families or phyla in the animal kingdom.

The axopods themselves carry kinetosomes, or little sticky-stabby organelles that act in catching prey. The long, fine axopods extend quite far beyond the cell, and wait until a hapless flagellate comes swimming by. Once triggered, the kinetosomes stick to and stun the victim, and other axopods are recruited to help hold it down. Typically, as one images, these flagellates are small, but it's not uncommon to see centrohelids with prey larger than themselves. In some species, the individuals even join forces (ie, fuse) with their neighbours to catch large beasts! (Sakaguchi et al. 2002 Eur J Protistol) The victim is then delivered towards the cell body proper, where pseudopods extend and engulf the food for a gradual digestion ritual. The waste is delivered outwards by the same axopods, which essentially make the cell functionally bigger without having to sustain all the material in-between that clogs up valuable gas exchange surface.

Centrohelids do move about, and in plenty of different ways: many float, some seem to use their axopods to glide along the surface, some attach to the surface with one end and slide like amoebae, some others apparently roll around like balls (Zlatogursky pers. comm.). They reproduce by cell division, like most things, but the complete life cycle remains unknown. To my knowledge, no flagellate stages have been found, but that does not mean they don't exist. To this day we keep on discovering new life cycle stages, even of well-known organisms, and quite often the two stages were long thought to be completely unrelated things. (a notorious case of this is in fungal taxonomy, of moulds, where the sexual and asexual fruiting bodies still carry separate genus and species names, primarily to deter taxonomically-challenged undergrads from pursuing mycology any further). I had the fortune to come across one in the midst of division, which you can see below:

Not much is known about how centrohelids live (let alone their potential molecular peculiarities), and the entire centrohelid research community at the moment constitutes two or three people. The field is so tiny that it was struck badly by an unfortunate unexpected death of their recent expert, Kiril Mikrjukov. With his passing, centrohelid research essentially stopped, and much valuable insight was lost. I know of at least one person currently trying to revive the centrohelids (to our eyes; they're doing quite fine without being noticed by us, of course), but that's it. I'm bringing this up to point out how tiny some areas of research are, and how easily they can be devastated by the loss (or stalled by the retirement) of one or two people. And since the old apprenticeship model where a student follows their advisor's footsteps is largely gone (in large part due to the horrid job market, rather than being replaced by something better), it's painful to think about the wealth of knowledge and insight that gets lost without ever seeing the light of publication. Many tantalising observations never make it to publication, sometimes due to experimental difficulties, but usually due to the limits of how much one individual can really do (not much, in science...), oh, and, of course, lack of funding. This is just one example, but the world of protistology is full of fascinating organisms left obscure and untended by human curiosity.

The views expressed are those of the author and are not necessarily those of Scientific American.

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